The present invention relates to centrifugal atomizers, preferably in the art of spraying liquid coatings, and including centrifugal atomizers incorporating electrostatic techniques in their operation.
Centrifugal atomizers are particularly adaptable for applying certain liquid coating materials, as the atomizers may be designed to create very small liquid particles or droplets, and such fine droplets create a very smooth coating finish. Centrifugal atomizers using electrostatic techniques offer further advantages in particle size control, and in increased efficiencies of applying the coating material which is atomized under the influence of centrifugal and electrostatic forces.
Centrifugal atomizers are conventionally designed in either of two atomization head configurations. In one configuration, the atomization head takes the form of a disk or a dish-shaped device, and in the other case the atomization head takes the form of a cup or bell-shape. In both cases, the atomizing head is affixed to a shaft which is rotatable at relatively high rotating speeds by a rotating drive mechanism. In both cases the coating material or other liquid is metered onto the rotating atomizing head surface, where it is centrifugally, and in some cases electrostatically, propelled from the surface toward an article to be coated. A coating material feed tube or passage is coupled to a source of coating material and leads to an opening proximate the atomizing head, so that coating material which is metered through the tube or passage is captured by the rotating surface, wherein the centrifugal forces may be applied to it.
The atomizing head of the centrifugal atomizer may be rotated at speeds ranging from a few thousand revolutions per minute (rpm) to in excess of 70,000 rpm. Coating material or liquid which is applied to the rotating surface becomes evenly distributed over the entire surface and centrifugal forces cause a generally radial flow outwardly toward an outer edge of the atomizing head. At the outer edge of the atomizing head the coating material is changed from a thin liquid film into a fine droplet cloud, and this cloud of droplets is directed toward the article to be coated. Electrostatic forces may be utilized to assist in the directional motion of the particles, or external air jets may be used for the same purpose.
Because of the high rotational speeds of the atomization head, air movement is created in the region proximate the front of the atomization head. This air moves about the axis of the atomization head in a cyclonic manner, thereby creating at the center region of the atomization head at least a partial vacuum, or a region of reduced pressure. This partial vacuum tends to draw some atomization particles into the center region of the atomization head, and the particles become deposited upon the central front face of the atomization head. If these particles are permitted to collect and to become dry, they will eventually break away from the center region and become incorporated into the atomization spray which is directed toward the article to be finished. If these dried particles are deposited upon the article they create flaws in the finish coating. It is therefore desirable that the atomization head of a centrifugal atomizer be designed so as to prevent the accumulation of dried particles on the central face of the atomization head, to eliminate the possibility of such particles becoming ultimately deposited upon the article to be finished.